Microwave filters



Oct. 9, 1962 zz ETAL 3,058,072

MICROWAVE FILTERS Filed Nov. 15, 1956 2 Sheets-Sheet 1 /NPU T POP T 22 camp/ FREQUENCY OF THE sER/E'S ARMS fi lO/fk =RESONAN7 FREQUENCY OF ms SERIES ARMS ATTENUAT/ON (db) .\0 B 8 6 8 //v l/EN TOPS PETER A. R/zz/ GEORGE C. SHA w ATTORNE Oct. 9, 1962 P. A. RIZZI ETAL MICROWAVE FILTERS 2 Sheets-Sheet 2 Filed Nov. 15, 1956 w m m 0 3Q 29% wbmtw Fla 8 OUTPUT PORT /Nl/EN7'ORS PETER A. 9122/ GEORGE C. SHAW BY 32mg TORNEY United States Patent 3,058,072 MICRGWAVE FILTERS Peter A. Rizzi, Providence, R.I., and George C. Shaw, Waltham, Mass., assignors to Raytheon Company, a corporation of Delaware Filed Nov. 15, 1956, Ser. No. 622,467 4 Claims. (Cl. 33373) This invention pertains to high-frequency electrical filters, and more particularly to low-pass and band rejection microwave filters for use in microwave transmitting and receiving systems.

In microwave filters of presently-used designs, the problem of obtaining a satisfactory VSWR (voltage standing wave ratio) in the pass band is inextricably bound to the problem of obtaining good rejection in the stop band. It is an objective of this invention to provide a microwave filter of a design permitting the problem of obtaining a satisfactory VSWR pass band to be separately treated from the problem of obtaining good rejection in the stop band. This result is achieved by coupling to a main wave guide section one or a number of cutoff filter sections which in essence are wave guide sections having a higher cutoff frequency than the main wave guide and which are terminated in such a manner as to reflect or absorb the microwave energy in the undesired frequency band.

The present invention also permits the construction of a wave guide filter having good band-rejection characteristics and obviates the need to introduce into the wave guide any extraneous materials, either insulative or conductive.

A standard rectangular wave guide, propagating the TE mode, has been selected as the basis to which are applied the principles of the invention. The application of such principles to other forms of wave guides and other modes of propagation will be obvious to those familiar with the microwave filter art. The invention, both as to its embodiments and method of operation, together with its advantages, will be better understood by reference to the following verbal exposition taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a perspective view of an embodiment of the invention employing a single stage series-T arm;

FIG. 2 is a sectional view of an actual filter taken along the line XX of FIG. 1;

FIG. 3 is a representative attenuation curve for the embodiment of FIG. 1;

FIG. 4 is a perspective view of an embodiment of the invention employing a single stage shunt-T arm;

FIG. 5 exemplifies one manner in which a number of filter arms may be cascaded;

FIGS. 6 and 7 are perspective views of embodiments of the invention employing the isolating properties of a magic-T junction; and

FIG. 8 is an attenuation curve typifying the results obtainable with the embodiment of FIG. 7.

Preliminarily, one must recognize that a closed wave guide is inherently a high-pass filter because of its ability to pass all frequencies above a cutoff frequency fc. Maxwells field equations show that an electromagnetic wave will propagate unattenuated in a closed wave guide only if its frequency is above the frequency fc. For the case of a rectangular wave guide operating in the dominant mode (TE the cutoff frequency fc may be calculated from the equation where A is the width of the wave guides broader wall, ,a is the permeability of the medium filling the wave guide,

and e is the mediums dielectric constant.

For frequencies below fc, an electromagnetic wave will attenuate at the rate of ,8 nepers per meter where a er e Thus a rectangular Wave guide (or any closed wave guide) is, by virture of the frequency cutoff effect, a highpass filter. Consequently, where a high-pass filter is required to pass frequencies above f one merely selects the dimension A of the rectangular wave guide to cause f to be the cutoff frequency. It is obvious from Equation 2 above that the length of the cutoif wave guide section determines the degree of attenuation for frequencies below h.

It is to be understood that FIGS. 1, 4, 6 and 7 are symbolic inasmuch as the thickness of the Wave guide walls are indicated to be infinitesimal whereas the walls of a practical wave guide are of finite thickness; hence, all dimensions indicated in those figures are in a practical wave guide to be measured between internal surfaces or points. This can be more fully appreciated by a collation of FIGS. 1 and 2. FIG. 1 is a symbolic representation of a low-pass microwave filter utilizing the inherent filtering attributes of a closed wave guide. FIG. 2 is a section view of a practical wave guide taken along the line XX of FIG. 1. Note that where the dimensions a, A and d are indicated externally in FIG. 1, the dimentions in FIG. 2 are measured internally. It will be noted that the dimension d in FIG. 2 is measured from the internal surface of the short circuit to a point within the main wave guide. Those familiar with the microwave art will understand that the point within the main wave guide represents the apparent input terminal to the arm 2. Distance d in FIG. 1, however, is symbolically represented as measured from short circuit 4 to the junction with the main Wave guide.

The series-T cutoff filter depicted in FIG. 1 consists of a hollow rectangular main wave guide 1 to which is coupled an arm 2 located on the broad wall 3 of the main wave guide. The arm 2 is itself a section of hollow rectangular wave guide but, as is evident from the drawing, the Width a of the arm 2 is smaller than the width A of the main wave guide 1. The direction of the electric field in the main Wave guide is indicated by the arrow E; therefore, arm 2 forms an E-plane T-junction so that the impedance of the arm is effectively in series with the impedance of the main wave guide. Because the dimen sion a of a series arm 2 is smaller than the width A of the main guide, it necessarily follows that the cutoff frequency of the series arms is higher than the cutofi frequency of the main wave guide 1. By suitable choice of dimension a, the arm 2 is made such that it is below cutoff for all frequencies which it is desired to pass. At frequencies below the cutoff frequency f of the series arm and above the cutolf frequency h of the main guide 1, the wave energy is transmitted from the input port to the output port with practically no loss. This result obtains because for frequencies below f the series arm arm 2 acts as a small reactive element in series with the main guide 1 rather than as a transmission line. Where the frequency of the wave energy is above the cutoff frequency f of the series arm, the arm acts like a normal series T. A shorting plate 4 is placed across the series arm 2 at a distance d from the junction. Since the series arm 2 is short-circuited a distance d from its input terminal, the filter section rejects frequencies at which d is approximately one-quarter wave length or an odd multiple thereof. By dimensioning the series arm so that its cutoff frequency is near to the resonant frequency of the short circuited series arm, an attenuation curve is obtained which rises very rapidly with frequency. A typical attenuation curve for a single stage series-T type filter 3 is shown in FIG. 3 where the ratio of the test frequency f to the cut off frequency of the series arm f is plotted against attenuation in decibles (db).

While the invention has been described with reference to a series-T arm, it is also feasible to construct a microwave filter which can perform the identical function by substituting a shunt-T arm 5 in the manner depicted in FIG. 4. The hollow rectangular shunt-T arm is located on the narrow wall 6 of the main guide 1 and forms an H-plane T-junction. The shunt arm is terminated by a short circuiting plate 7 which is positioned a distance it from the junction. Since the impedance of the shunt arm 5 is effectively in parallel with the main guide impedance, the filter section rejects frequencies at which 11 is approximately one-half wave length or an even multiple thereof.

It has been experimentally determined with a filter of the series-T type depicted in FIG. 1 that at frequencies below f the voltage standing wave ratio (VSWR) is practically constant. This fact, and the fact that in the filters pass band the series arm 2 is below cutoff, are important considerations because they permit the problem of obtaining a good VSWR in the pass band and good rejection in the stop band to be treated separately. Consequently, a filter may be constructed employing a plurality of series-T arms, as illustrated in FIG. 5. The arms, 8, 9, 10 and 11 are spaced to give a good impedance match across the pass band, and the shorting plates 12, 13, 14 and 15 terminating the arms are positioned to effect good rejection in the stop band. The positioning of the shorting plates does not in any way affect the impedance match in the pass band. In the embodiment of FIG. 5, the four E-plane T arms are dimensioned to have the same cutoff frequency f Arms 8 and 9 are tuned to a first resonant frequency, and arms 10 and 11 are tuned to a second resonant frequency, to effect an enlarged stop band. By spacing the first two arms 8 and 9 one-quarter of a wave length (M4) apart, where A is the wave length at a selected frequency in the pass band, the reflections due to their small reactive effect tend to cancel. For the same reason, M4 spacing for the next two arms 10 and 11 also causes cancellation of reflections. Finally, by spacing the two pairs apart, as indicated in FIG. 5, second order cancellation is obtained so that at frequencies in the pass band where the mismatch of each pair is large, the AA spacing tends to cause cancellation of reflection and thereby maintains the VSWR in the pass band fairly low for the complete filter. It is emphasized here that a plurality of arms may be arranged in various combinations to effect a satisfactory impedance match across the pass band and that the arrangement of FIG. 5 is intended merely to illustrate one such arrangement. It should also be apparent to the reader that shunt-T arms of the type shown in FIG. 4 may be assembled in various arrangements in an analogous manner to effect an enlarged stop band.

Another embodiment of the invention is illustrated in FIG. 6. That embodiment employs a length of rectangular wave guide having an E-plane T-arm and an H-plane T-arm added in the manner of a magic-T. A magic-T, as is well known, consists of two branches or arms of straight hollow wave guide of rectangular cross section, disposed at right angles to each other, joined to a main section of similar wave guide at an intermediate common junction point so as to form in the E-plane the equivalent of a series electrical connection and in the H-plane the equivalent of a parallel electrical connection. Because of the dissimilarity in the magic-T of the electrical connections of the two arms with the main wave guide, one being in series and the other in parallel, the two arms are conjugate or in balanced electrical relation with respect to each other, and the two collinear portions of the main wave guide disposed on opposite sides of the common junction point are also conjugate with respect to each other and are in unbalanced electrical relation with respect to each of the two arms. The embodiment of FIG. 6 differs from the magic-T, however, in that the width of the E-arm '16 and the H-arm 17 has been reduced from the width A to the width at, and the arms 16 and 17 are terminated by short circuiting plates 18 and 19 respectively. At frequencies below the cutoff frequency f of the E- and H-arms, the energy introduced at the input port propagates through the main guide 1 to the output port with practically no loss. The E- and H-arms 16 and 17 have only a small reactive effect at those lower frequencies and do not act as transmission lines. The VSWR due to the small reactive effect can be reduced by placing a small capacitive button in the main guide at the center of the magic-T junction as indicated by the phantom button 20 in FIG. 6. Since the mismatch is corrected in this manner at the source of the discontinuity, no long line lengths are involved and consequently a low VSWR is maintained over a considerable pass band.

Due to the action of the magic-T, frequencies higher than f divide equally between the E-arm and the H- arm. The electrical distance L from the input junction to the short circuit is identical for the E- and H-arms. Because the distance L is electrically identical for both arms and by virtue of the phase relations in a magic-T, the phase of the energy reflected from the E- and H-arms is such that it is additive in the branch 21, but in the branch 22 the reflected energy from the E-arm cancels the energy reflected from the H-arm. The sole other requirement on the distance L, in addition to the requirement that it be electrically identical for the E- and H-arms, is that the distance be long enough to attenuate frequencies below the cutoff frequency of the E- and H-arms before the energy reaches the shorts. Usually 10 db attenuation is adequate, and the required length for any attenuation can be calculated from Equation 2.

To obtain greater rejection in the stop band, two or more magic-T cutoff filters may be cascaded, or a double magic-T cutoff filter may be constructed by employing two E-arms and two H-arms at the same junction.

FIG. 7 depicts a modification of the magic-T cutoff filter shown in FIG. 6. It has been found in practice that wide-band rejection of the magic-T cutoff filter of FIG. 6 can be improved, and the reason that an improvement may be obtained is that when the electrical lengths from the input terminals to the short circuits in the E- and H-arms are equal, the actual lengths of the E- and H arms are not equal. The electrical length of the E- arm is determined by the actual length of the E-arm plus the distance from the top wall of the main guide to the apparent input terminal, which is located approximately one-quarter of the guide height down from the top wall. The electrical length of the H-arm is a combination of the actual length of the shorted H-arm plus the distance from the side wall to the apparent input terminal of the shunt arm which is located at approximately the center of the main guide. Because the apparent input terminal of the E,-arm is distinct from the apparent input terminal of the H-arm, different lengths of E- and H-stubs are required to obtain the same electrical length from the apparent input terminals to the short circuits. As a consequence of making the E- and H-arms of identical electrical length, the frequency sensitivity of the arms is different (due to dissimilar actual lengths), and adjustment of the line lengths for good rejection at one frequency will not necessarily result in good rejection at all frequencies in the stop band. To compensate for this effect, either the E- or H-arm may be compounded of two wave guide sections of different broad wall widths which will tend to cancel the difference in frequency sensitivity. In the embodiment of FIG. 7, the E-arm is a compound of two wave guide sections 23 and 24. The section 23, which is coupled directly to the main guide 1, has a width equal to the width a of the H-arm 25. The upper section 24 of the E-arm has an enlarged broad wall of width 122.

The dimensions of section 24 may be determined by empirical means. It is to be noted that the lengths L of the E- and H-arms are electrically identical; that is, the combined height of sections 23 and 24 is always maintained equal to L.

- FIG. 8 depicts an attenuation curve showing the results actually obtained with a microwave filter of the type illustrated in FIG. 7. In comparing the curve of FIG. 8 with the curve of FIG. 3, it is evident that the rejection band obtained with a magic-T filter type is much broader than the rejection band which can be obtained with a single stage series-T filter.

While the description of the invention has thus far been restricted to microwave filters of the energy-reflection type, the embodiments of the invention also encompass microwave filters of the energy-absorption type. For example, the magic-T cutofi filter of FIG. 6 has been described as having E- and H-arms terminated in short circuits, but if an absorption-type filter is desired, the short circuits are replaced'by resistive loads having values equal to the characteristic impedance of the E- and H- arms. The energy proceeding into the E- and H-arms when so terminated is absorbed, and substantially none of the energy is reflected.

The embodiments shown in FIGS. 1 and 4 may similarly be converted to absorption filters by replacing the short-circuiting plates with resistive loads having values equal to the characteristic impedance of the E- and H- arms. In this connection, it should be observed that where a resistive load of characteristic impedance is substituted for the short-circuiting plate, the arm is no longer a resonant element, and hence the length of the arm, be it a one-quarter wave length or a one-half wave length or some other value, is of no material significance, since the arm may be of any convenient length and still efiectively absorb energy.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention Within the art.

What is claimed is:

1. A microwave filter comprising a main wave guide of rectangular cross-section having an input port and an output port, said main wave guide having a cutoff frequency 3, a pair of similar wave guide arms of substantially constant rectangular cross-section having a cutoff frequency f which is of higher frequency than f said arms being coupled directly to said main Wave guide in the manner of a magic-T whereby one of said arms forms an E-plaue T junction, and the other of said arms forms an H-plane T junction, and means directly terminating each of said arms in its characteristic impedance, thereby to prevent frequencies above f impressed at said input port from arriving substantially unattenuated at said output port.

2. A microwave filter comprising a main wave guide of rectangular cross-section having an input port and an output port, said main wave guide having a cutoff frequency h, a pair of similar wave guide arms of rectangular crosssection having a cutoff frequency f which is of higher frequency than f said arms being coupled to said main wave guide in the manner of a magic-T whereby one of said arms forms an E-plane T junction, and the other of said arms forms an H-plane T junction, and means terminating each of said arms in a short circuit, said arms being of identical electrical length.

3. An absorption-type microwave filter comprising a main wave guide of rectangular cross-section having an input port and an output port, said main Wave guide hav ing a cutoff frequency 1, a pair of wave guide arms of rectangular cross-section having a cutoff frequency f which is of higher frequency than f said arms being coupled to said main wave guide in the manner of a magic- T whereby one of said arms forms an E-plane T junction, and the other of said arms forms an H-plane T junction, and resistive means terminating each of said arms in its characteristic impedance, the electrical length of said arm-s being identical.

4. A microwave filter comprising a main wave guide of rectangular cross-section having an input port and an output port, said main wave guide having a cutoff frequency h, a pair of wave guide arms of rectangular cross-section having a cutoff frequency f which is of higher frequency than f said arms being coupled to said main wave guide in the manner of a magic-T whereby one of said arms forms an E-plane junction and the other of said arms forms an H-plane junction, said arms being of identical length, one of said arms being compounded of joined wave guide sections having different broad wall widths, and short-circuiting means terminating each of said arms.

References Cited in the file of this patent UNITED STATES PATENTS 2,531,447 Lewis Nov. 28, 1950 2,588,226 Fox Mar. 4, 1952 2,649,576 Lewis Aug. 18, 1953 2,785,381 Brown Mar. 12, 1957 OTHER REFERENCES Ragan: Microwave Transmission Circuits, vol. 9, M.I.T. Radiation Laboratory Series (pages 643-645 es pecially of interest), copyright May 21, 1948, McGraw- Hill Book Co., NY.

Rizzi: Microwave Filters Utilizing the Cutoff Effect, IRE Transactions, Microwave Theory and Techniques, vol. MTT-4, January 1956, No. 1, pages 36-40. 

